The invention relates to a method and an arrangement for arranging traffic in a telecommunications system, and particularly in a telecommunications system wherein data is transmitted cyclically.
An example of a telecommunications system wherein data transmission takes place cyclically according to certain cycle times is a common industrial automation system comprising a programmable logic controller (PLC or a “SoftPLC”, which refers to replacing an execution unit of the programmable logic with an application program to be run in a computer to execute commands of the logic program), related actuators and sensors or other devices and a bus connecting the same, e.g. a Profibus or the like. Typically, the internal telecommunication of such a system is cyclic such that the transmitter of data, e.g. a PLC, transmits a certain (same) message at certain intervals according to a corresponding cycle time, in other words a cycle time refers to a time between the starting moments of the transmission of two such successive messages. The message is delivered to a device connected to the bus, and the device typically replies by transmitting a reply message. When the cycle time is sufficiently short, in practice data may thus be transferred in real time e.g. between a PLC and an actuator, so that e.g. the real-time state information or the like of the actuator is available to the PLC. Such cycles usually occur several at the same time, each cycle having a certain cycle time. The cycle times of different cycles may differ from one another or they may be equal in length. Furthermore, in such a system, a message cycle is usually always initiated by a device operating as a master, such as a PLC, while other devices (slave devices) only reply to the messages received from the master device. In such a case, each message cycle has a predetermined transfer time on a data transmission path, thus enabling collisions between different messages, i.e. simultaneous transmissions on to the data transmission path, to be prevented.
In some processes, however, a need also exists to communicate from one slave device to another, i.e. slave-to-slave communication is needed. Typically this takes place through a master device such that the master device asks a first slave device for the necessary information and writes the received reply to a second slave device. A problem with the above-described solution is that such a procedure unnecessarily consumes the capacity of the data transmission path used as well as the processor capacity of the master device since communication between slave devices always takes place through a master device.